483

On Emma and Pat's first full day in the Mara, we had the incredibly opportunity to witness a wildebeest crossing that resulted in some success and some tragedy.

One of the focuses of my dissertation research is the occasional mass inputs of nutrients to the river caused by drownings of wildebeest during crossings. Although it is not uncommon for a few wildebeest to die in every crossing, occasionally a mass drowning event will claim the lives of hundreds or thousands of individuals. These drowning events may be due to high water levels, the size of the herd crossing, interference by tourist vehicles at the exit, or simply a bad decision about where to cross. These large drowning events have been noticed for years, and they have been studied by paleoecologists interested in their potential role in providing meat to early humans, but no one has really quantified how large these events can be and how these pulse inputs may affect the river ecosystem, which are the focuses of my research.

Although I have read about these drowning events and seen their aftermath, I had not actually witnessed one occur until this summer, and it helped me gain a better understanding of how these events can happen. However, seeing a number of carcasses piled up is completely different than actually witnessing hundreds of animals panicked and drowning, and it was a completely heart-wrenching experience.

We had driven to the hippo pool near the lower Mara Bridge (aka Purungat Bridge) to show Emma and Pat one of our focal hippo pools, and we saw several thousand wildebeest massed on the far side of the bank. As they tend to do before crossing the river, they were milling around anxiously, occasionally approaching the water and then quickly retreating, each one seeming to come forward to test its bravery and then getting cold hooves. Finally, one brave individual leapt into the river and started swimming across, and it was like a cork was popped, and the rest of the herd started flowing into the river.

Wildebeest crossing the Mara River

If they had crossed directly at the hippo pool, there would have been an easy path out. However, they seemed to be intimidated by the hippos and moved upstream a bit before crossing, and across the river from their point of entry was a fairly steep bank with only one narrow path up. As the herd continued crossing, frantically inspired by their need to follow one another and blind to the other side of the river, hundreds of individuals began piling up on the far side of the river. They either found themselves waiting in a long line to slog up an increasingly slippery path, perched on a low shelf with no path to the upper bank, or trying to keep themselves afloat in the deep waters of the pool.

Wildebeest unable to get out of the river

As more and more began massing in the water, we could start to hear their grunts and cries. Although they were a bit upstream from us and shielded from view, we could see through binoculars as they strained their nostrils above water. After several more minutes, we began to see their bodies float downstream. It was just a few at first, but they continued steadily, and in total we counted 483 carcasses. We estimated around 2,000 individuals had tried to cross, meaning nearly a quarter of this herd had perished in a single crossing event.

Wildebeest carcasses floating downstream

Some of the herd did eventually figure out that if they swam downstream a bit, they could exit easily through the hippo paths. However, the hippos became increasingly irritable at this invasion into their territory and aggressive towards the intruders. Several juvenile wildebeest were actually chased out of the pool onto the far bank several times, despite their repeated attempts to re-join the herd.

A hippo being aggressive toward a wildebeest

Hippos chasing a wildebeest out of the river

Anticipation of this aggressive behavior may have been one reasons the wildebeest didn't cross directly through the pool in the first place, despite the easier exit on the far side. Ultimately, these drowning events are likely caused by a confluence of factors. For example, at lower water levels, such as we saw in 2009, the wildebeest can simply stand on the river bottom while they sort out their exit path. Similarly, the effects of the drowning events on the river are influenced by a number of factors, which makes it important for us to study these rare events whenever they occur. Unfortunately, we weren't able to document any effects from this event, as almost all the carcasses floated directly into Tanzania, a macabre completion of the migration loop.

This was the third large drowning of the year. On Aug. 28-29, we estimate around 500 drowned several kilometers upstream of here. As we were counting those carcasses on Aug. 31, we witnessed another crossing at a bad location in which ~1,000 drowned. Including this most recent crossing, the total number of drownings for this season thus far is 2,000-- a drop in the bucket against a herd of 1.3 million, but a profound loss nonetheless.

You can watch a video Chris captured of much of the crossing here. The struggle against the far bank took place out of view upstream, but you can see some of the survivors emerging, too exhausted to feel triumphant, near the end of the video.